The fossil, found in the Gogo area of northwest Australia, is proof that an ancient species had advanced reproductive biology, comparable to modern sharks and rays, said John Long, head of sciences at the Museum of Victoria in Melbourne.

"It is not only the first time ever that a fossil embryo has been found with an umbilical cord, but it is also the oldest known example of any creature giving birth to live young," Long told Reuters.

The placoderms, often referred to as "the dinosaurs of the seas", were the rulers of the world's lakes and seas for almost 70 million years. Most species of the armored fish were quite small but some reached over 20 feet in length.

Placoderms are from the late Devonian period when land animals evolved from fish.

"This discovery changes our understanding of the evolution of vertebrates," Long said.

"It will make us rethink the early evolution of vertebrate in terms of how reproduction has driven evolutionary events."

Long said little was known about how reproductive changes from spawning eggs to internal fertilization affected the evolution species.

The scientists have published their finding in the latest Nature journal (http://www.nature.com/nature).

"The new specimen, remarkably preserved in three dimensions, contains a single, intra-uterine embryo connected by a permineralized umbilical cord. An amorphous crystalline mass near the umbilical cord possibly represents the recrystallized yolk sac," wrote the scientists.

"Our new example extends the definite record of vertebrate viviparity (giving birth to live young) back by some 200 million years," the scientists wrote in the journal.

They said the new discovery points to internal fertilization and viviparity in vertebrates as originating earliest within placoderms.

The Australian scientists have named their 25-cm fossil, Materpiscis attenboroughi, in honor of Sir David Attenborough, who first drew attention to the Gogo fish sites in the 1979 series Life on Earth.

"The existence of the embryo and umbilical cord within the specimen provides scientists with the first ever example of internal fertilization, sex, confirming that some placoderms had remarkably advanced reproductive biology," said Long.

"This is the first bit of evidence on how a complete extinct class of animals may have reproduced."

The fossil will go on display in the foyer of Melbourne Museum from May 29.